1. Field of the Invention
This invention relates to improvements in an exhaust gas purifying system including a trap filter for trapping particulates discharged from an internal combustion engine, and more particularly an exhaust gas purifying system in which a regeneration operation of the trap filter is effectively performed without causing thermal damage to the trap filter.
2. Description of the Prior Art
Most diesel engines are equipped with a trap filter for trapping particulates and the like which are discharged from an engine for the purpose of preventing them from being emitted to the ambient air. When the back pressure of the engine increases due to the accumulation of the particulates trapped by the trap filter, the particulates are periodically burned to regenerate the trap filter, thereby preventing the back pressure from affecting engine performance.
For the purpose of burning the particulates, unburned fuel containing burnable gases, such as hydrocarbon and/or carbon monoxide, is supplied to the upstream side of the trap filter and burned together with the particulates. During the burning, the trap filter may be thermally damaged, and therefore the supply amount of fuel must be carefully controlled in order to avoid such damage.
In view of the above, it has been proposed to perform the regeneration of the trap filter by controlling the fuel supply amount to the trap filter in accordance with an exhaust gas temperature, upon judging a regeneration required time or timing a of the trap filter in accordance with accumulated values of engine speed. Such an arrangement is disclosed, for example, in Japanese Patent Provisional Publication No. 59-122721. With this arrangement, in an exhaust gas high temperature condition, the trapped particulates are naturally burned or securely burned with a small amount of fuel supplied to the trap filter. In an exhaust gas low temperature condition, the fuel supply amount is increased to effectively burn the trapped particulates.
However, the following difficulties have been encountered in the above discussed conventional arrangement. That is to say, the fuel supply amount required for the regeneration of the trap filter changes not only in accordance with the exhaust gas temperature but also in accordance with an intake air amount during a regeneration operation of the trap filter regeneration. More specifically, when engine speed decreases to reduce an exhaust gas amount during the trap filter regeneration operation, a fuel concentration in exhaust gas becomes excessively high thereby providing the possibility of the filter being thermally damaged or partially burned. When the exhaust gas amount is large in the low exhaust gas temperature condition in which the fuel supply amount is small, the trapped particulates cannot be effectively burned.
Thus, with the conventional arrangement, an appropriate regeneration operation cannot be achieved from the view points of trap filter regeneration efficiency and durability of the trap filter, in the situation where the fuel supply amount is controlled merely in accordance with the exhaust gas temperature at the start of the regeneration operation.